A catalytic converter includes a catalyst carrier that is formed of a ceramic or the like and supports a catalyst for purifying unburned hydrocarbons (HC), nitrogen oxides (NOx), and the like contained in exhaust gas, a casing that is formed of a metal or the like and houses the catalyst carrier, and a catalytic converter holding mat that is positioned in the gap between the catalyst carrier and the casing and retains the catalyst carrier.
The catalytic converter holding mat is provided to retain the catalyst carrier inside the casing. The catalytic converter holding mat is also used to prevent leakage of exhaust gas through the gap between the catalyst carrier and the casing. Specifically, the catalytic converter holding mat is required to exhibit a cushioning capability for protecting the catalyst carrier from vibrations applied from the outside, an exhaust gas sealing capability, and the like.
For example, Patent Document 1 discloses a catalytic converter holding mat that is used for an internal combustion engine exhaust gas purification apparatus wherein a retention mat that includes inorganic fibers is wound around the outer circumferential surface of a carrier, and the carrier is retained inside a case through the retention mat in a state in which the retention mat is compressed in the thickness direction. The retention mat that is used as a catalytic converter holding mat is provided with a high-density part over the entire circumferential surface in at least one part of the exhaust gas purification apparatus in the axial direction, the high-density part having a density higher than that of the remaining part.
Patent Document 2 discloses a catalytic converter holding mat that is provided in a catalytic converter that includes a tubular catalyst carrier, a casing that houses the catalyst carrier, and a catalytic converter holding mat that is fitted to the catalyst carrier and provided in the gap between the catalyst carrier and the casing, the catalytic converter holding mat being produced by molding inorganic fibers in the shape of a mat or a cylinder, and characterized in that the basis weight is set to be smaller than that of the remaining area on at least the exhaust gas entrance-side end over a specific length in the axial direction.